Shoes, and particularly athletic shoes, can be described as including two major categories of components namely, a shoe upper and a sole. The general purpose of the shoe upper is to snugly and comfortably enclose the foot. Ideally, the shoe upper should be made from an attractive, highly durable, yet comfortable material or combination of materials. The sole, which also can be made from one or more durable materials, is primarily designed to provide traction, and to protect the wearer's feet and body during any use consistent with the design of the shoe. The considerable forces generated during uses such as athletic activities require that the sole of an athletic shoe provide enhanced protection and shock absorption for the feet, ankles and legs of the wearer. For example, impacts which occur during running activities can generate forces of up to two to three times body weight; certain other activities, e.g., playing basketball, have been known to generate forces of up to approximately 6-10 times an individual's body weight. Accordingly, many shoes and, more particularly, many athletic shoe soles are now provided with some type of resilient, shock-absorbent material or shock-absorbent components to cushion the user during strenuous athletic activity. Such resilient, shock-absorbent materials or components have now commonly come to be referred to in the shoe manufacturing industry as the mid-sole.
More specifically, it has been a focus of the industry to seek a mid-sole design which achieves an effective impact response in which both adequate shock absorption and resiliency are appropriately taken into account. Such resilient, shock-absorbent materials or components could also be applied to the insole portion of the shoe, which is generally defined as the portion of the shoe upper directly underlining the plantar surface of the foot.
A specific focus in the shoe manufacturing industry has been to seek mid-sole or insert structure designs which are adapted to contain fluids, in either the liquid or gaseous state, or both. Examples of gas-filled structures which are utilized within the soles of shoes are shown in U.S. Pat. Nos. 900,867 entitled "Cushion for Footwear" which issued Oct. 13, 1908, to Miller; 1,069,001 entitled "Cushioned Sole and Heel for Shoes" which issued Jul. 29, 1913, to Guy; 1,304,915 entitled "Pneumatic Insole" which issued May 27, 1919, to Spinney; 1,514,468 entitled "Arch Cushion" which issued Nov. 4, 1924, to Schopf; 2,080,469 entitled "Pneumatic Foot Support" which issued May 18,1937, to Gilbert; 2,645,865 entitled "Cushioning Insole for Shoes" which issued Jul. 21, 1953, to Towne; 2,677,906 entitled "Cushioned Inner Sole for Shoes and Method of Making the Same" which issued May 11, 1954, to Reed; 4,183,156 entitled "Insole Construction for Articles of Footwear" which issued Jan. 15, 1980, to Rudy; 4,219,945 entitled "Footwear" which issued Sep. 2, 1980, also to Rudy; 4,722,131 entitled "Air Cushion Shoe Sole" which issued Feb. 2, 1988, to Huang; and 4,864,738 entitled "Sole Construction for Footwear" which issued Sep. 12, 1989, to Horovitz; all of which are incorporated herein by reference. As will be recognized by those skilled in the art, such gas filled structures (often referred to in the shoe manufacturing industry as "bladders") typically fall into two broad categories, namely (1) "permanently" inflated systems such as those disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 and (2) pump and valve adjustable systems as exemplified by U.S. Pat. No. 4,722,131. By way of further example, athletic shoes of the type disclosed in U.S. Pat. No. 4,182,156 which include "permanently" inflated bladders have been successfully sold under the trade mark "Air Sole" and other trademarks by Nike, Inc. of Beaverton, Oreg. To date, millions of pairs of athletic shoes of this type have been sold in the United States and throughout the world.
The permanently inflated bladders are typically constructed under methods using a flexible thermoplastic material which is inflated with a large molecule, low solubility coefficient gas otherwise referred to in the industry as a "super gas," such as SF.sub.6. By way of example, U.S. Pat. No. 4,340,626 entitled "Diffusion Pumping Apparatus Self-Inflating Device" which issued Jul. 20, 1982, to Rudy, which is expressly incorporated herein by reference, discloses a pair of elastomeric, selectively permeable sheets of film which are formed into a bladder and thereafter inflated with a gas or mixture of gases to a prescribed pressure which preferably is above atmospheric pressure. Ideally, the gas or gases utilized have a relatively low diffusion rate through the selectively permeable bladder to the exterior environment while gases such as nitrogen, oxygen and argon (which are contained in the atmosphere and have a relatively high diffusion rate) are able to penetrate the bladder. This produces an increase in the total pressure within the bladder by the additive nature of the partial pressures of the nitrogen, oxygen and argon which diffuse into the bladder from the atmosphere and the partial pressures of the gas or gases contained initially injected into the bladder upon inflation. This concept of an almost total "one-way" addition of gases to enhance the total pressure of the bladder is now known in the art as "diffusion pumping."
In a diffusion pumping system, there is a period of time involved before a steady state of internal pressure is achieved. The period of time is related to the bladder material used and the choice of gas or gases contained in the bladder. For example, oxygen tends to diffuse into the bladder rather quickly with the effect being an increase in the internal pressure of approximately 2.5 psi. In contrast, over the course of a number of weeks nitrogen gas will gradually diffuse into the bladder resulting in an increase of pressure to approximately 12.0 psi. This gradual increase in bladder pressure typically causes an increase in tension in the skin, resulting in a volume increase due to stretching. This effect is commonly referred to in the industry as "tensile relaxation" or "creep." Thus, the initial selection of materials employed in the bladder and the choice of the captive gas or gas mixture utilized to initially inflate the bladder is critical to achieving a bladder which is essentially permanently inflated at a desired internal pressure and which therefore maintains a desired internal pressure over an extended period of time.
Prior to and shortly after the introduction of the Air Sole.TM. athletic shoes, many of the mid-sole bladders employed in the industry consisted of a single layer gas barrier type film made from polyvinylidene chloride based materials such as Saran.RTM. (which is a registered trademark of the Dow Chemical Co.). These materials which, by their nature are rigid plastics, are less than ideal from the standpoint of flex fatigue, heat sealability, elasticity, and degradation. Attempts to address these limitations by creating bladder films made by techniques such as laminations and coatings (which involve one or more barrier materials in combination with a flexible bladder material such as various thermoplastics) then present a wide variety of problems typical of such combinations. Such difficulties with composite constructions typically include layer separation; peeling; gas diffusion or capillary action at weld interfaces; low elongation which leads to wrinkling of the inflated product; cloudy appearing finished bladders; reduced puncture resistance and tear strength; resistance to formation via blow-molding and/or heat-sealing and/or R-F welding; high cost processing; and difficulty with foam encapsulation and adhesive bonding; among others.
The art has attempted to address these problems (created by trying to laminate two or more dissimilar materials to balance the advantages and disadvantages of any single material) by the use of tie-layers or adhesives between the layers in preparing laminates. The use of such tie layers or adhesives help solve some of the difficulties noted above but generally prevent regrinding and recycling of any waste materials created during product formation back into an usable product, and thus, also contribute to high cost of manufacturing and relative waste. These and other short comings of the prior art are described in more extensive detail in U.S. Pat. Nos. 4,340,626; 4,936,029 and 5,042,176 which are hereby expressly incorporated by reference.
With the extensive commercial success of the products such as the Air Sole.TM. shoes, consumers have been able to enjoy a product with a long service life, superior shock absorbency and resiliency, reasonable cost, and inflation pressure stability, without having to resort to pumps and valves. Thus, in light of the significant commercial acceptance and success that has been achieved through the use of long life inflated gas filled bladders, it is highly desirable to develop advancements to solve the few remaining disadvantages associated with such products. The goal then is to provide flexible, "permanently" inflated, gas-filled shoe cushioning components which meet, and hopefully exceed, performance achieved by such products as the Air Sole.TM. athletic shoes offered by Nike, Inc.
One key area of potential advancements stems from a recognition that it would be desirable to employ "capture" or "captive" gases other than the large molecule, low solubility coefficient "super gases" as described in the '156, '945 and '738 patents, replacing them with less costly and possibly more environmentally friendly gases. For example, U.S. Pat. Nos. 4,936,029 and 5,042,176 specifically discuss the methods of producing a flexible bladder film that essentially maintains permanent inflation through the use of nitrogen as the captive gas. As further described in U.S. Pat. No. 4,906,502, also expressly incorporated herein by reference, many of the perceived problems discussed in the '029 and '176 patents are solved by the incorporation of mechanical barriers of crystalline material into the flexible film such as fabrics, filaments, scrims and meshes. Again, significant commercial success for footwear products using the technology described in '502 patent (marketed under the trademark Tensile Air.TM. by Nike, Inc.) has been achieved. The bladders utilized therein are typically comprised of a thermoplastic urethane laminated to a core fabric three-dimensional, double bar Raschel knit nylon fabric, having SF.sub.6 as the captive gas contained therein. As discussed in the '502 patent, such bladder constructions have reduced permeability to SF.sub.6, nitrogen and other captive gases.
Exemplary of an accepted method of measuring the relative permeance, permeability and diffusion of different film materials is set forth in the procedure designated as ASTM 1 434V. According to ASTM 1 434V, permeance, permeability and diffusion are measured by the following formulas:
Permeance ##EQU1## Permeability ##EQU2## Diffusion ##EQU3##
By utilizing the above listed formulae, the gas transmission rate in combination with a constant pressure differential and the film's thickness, can be utilized to define the movement of gas under specific conditions. In this regard, the preferred gas transmission rate (GTR) for a bladder in an athletic shoe component which seeks to meet the rigorous demands of fatigue resistance imposed by heavy and repeated impacts has a gas transmission rate (GTR) value of less than about 10, more preferably less than about 7.5, still more preferably less than about 5.0, and still more preferably, a (GTR) value of 2.0 or lower, preferably as measured by the above procedure.
In addition to the aforementioned, the '029 and '176 patents also discuss problems encountered with previous attempts to use co-laminated combinations of plastic materials at least one of which is selected to operate as a barrier to oxygen. In this regard, the principal concern was the lack of fatigue resistance of the barrier layer. As described in the '176 patent, a satisfactory co-lamination of polyvinylidene chloride (such as Saran.RTM.) and a urethane elastomer would require an intermediate bonding agent. Under such a construction, relatively complicated and expensive processing controls would be required, such as strict time-temperature relationships and the use of heated platens and pressures, coupled with a cold press to freeze the materials together under pressure. Additionally, using adhesive tie layers or incorporating crystalline components into the flexible film at high enough levels to accomplish a gas transmission rate of 10 or less, dramatically reduces the flexibility of the film.
It is therefore, a principal object of the present invention to provide an inflatable cushioning device that is essentially permanently inflated with nitrogen or another environmentally desirable gas or combination of gases which meet the goals of flexibility, durability and low cost.
It is another object of the present invention to provide a cushioning device having a permeable barrier material with a gas transmission rate value of 10.0 or less.
It is still another object of the present invention to provide a cushioning device which substantially resists peeling between layers, is relatively transparent and economical to manufacture.
It is yet another object of the present invention to provide a cushioning device where the barrier layer substantially resists delamination and does not require a tie layer between the barrier layer and the flexible layers.
It is a further object of the present invention to provide a cushioning device which is formable utilizing the various techniques including, but not limited to, blow-molding, tubing, sheet extrusion, vacuum-forming, heat-sealing and RF welding.
It is an additional object of the present invention to provide a gas cushioning device which prevents gas from escaping along interfaces between the materials via capillary action.
It is yet another object of the present invention to provide a cushioning device which allows for normal footwear processing such as encapsulating the cushioning device in formable material.
The above list is not intended to be exhaustive of the objects or advantages of the present invention.